A circulating fluidized bed (CFB) boiler is a device for generating steam by burning fossil fuels in a combustion chamber operated under a special hydrodynamic condition. The CFB technique is commonly implemented in combustion and gassing processes. The essential advantage of the circulating fluidized bed technique in comparison with other reaction types is the excellent material and heat transfer between the particles and the gas. By using a sufficient gas velocity, a nearly isothermic state is produced in the reactor. This essentially facilitates the managing of the combustion and gassing processes.
CFB boilers can be briefly characterized as follows. Several tons of fine solid particles (e.g., sand and ashes) with a small addition of fuel particles are suspended in a powerful primary air stream blown from the bottom of the boiler. If the air velocity is chosen properly, the solid particles dragged by the gas stream exhibit behavior very similar to a boiling liquid. This phenomenon achieved by the primary air stream is called fluidization and the suspended material is referred to as the fluidized bed. At the same time, the fuel particles are burnt in these conditions in order to generate heat captured by water to produce steam. The fuel has to be supplied continuously to continue the operation.
Fluidized bed combustors are distinguished by low emissions and their capability to burn fuels of low or variable quality, such as turf or lignite. The reason is the fluidization conditions allow low combustion temperatures (e.g., approximately 800-900 deg of Celsius) under which almost no nitrogen oxides emissions arise. Also, the low temperatures and slow combustion allow the limestone to be added to the bed to capture sulfur oxides effectively. On the other hand it is assumed the CFB boilers are difficult to change their thermal power abruptly. This limits their use as it is often required to change the boiler thermal power according to varying load in the electrical grid.
It is therefore believed that a need exists for an improved control method and system for achieving an enhanced dynamic response of the CFB boiler load, as is disclosed in greater detail herein.